Magnetic circuit for an electro-acoustic converter

ABSTRACT

An improvement in an electro-acoustic converter having a magnetic anisotropic circuit with air gap therein, the improvement including an air gap formed of a magnetic material laminated with an insulating layer having a comparatively higher magnetic resistance in the magnetic circuit, thereby increasing a magnetic resistance against a magnetic flux caused by a voice coil and acting so as to reduce eddy current losses, and thus eliminating the reproduction distortion caused by the magnetic non-linearity of the magnetic circuit.

United States Patent H91 Yamamuro Feb. 18, 1975 1 1 MAGNETIC CIRCUIT FORAN 3,665,352 5/1972 Dietrich a a1 179/119 ELECTRO ACOUSTIC CONVERTER3,783,311 l/l974 Sato et a1. 179/115.5 R

Isao Yamamuro, Tokorozawa, Japan Inventor:

Pioneer Electronic Corporation, Tokyo, Japan Filed: Dec. 18, 1972 Appl.No.: 315,970

Assignee:

Foreign Application Priority Data Dec. 17, 1971 Japan 46-102518 US. Cl.179/115.5 R, 179/119 R Int. Cl H041 9/02 Field of Search l79/1l5.5 R,115.5 VC, 179/117,119 R; 335/231, 236, 296, 301,

References Cited UNITED STATES PATENTS 10/1934 Brzeski 179/1 15.5 R6/1943 Blessing 179/1 15.5 R l/l972 Mauz 179/1 15.5 R

INSULATING LAYERS OTHER PUBLICATIONS Brailsford, Magnetic Materials,1960, Methuen and Co., Chaps. 4 and 5, pp. 84-136.

Primary ExaminerWilliam C. Cooper Assistant Examiner-George G. StellarAttorney, Agent, or FirmSughrue, Rothwell, Mion, Zinn & Macpeak [57]ABSTRACT coil and acting so as to reduce eddy current losses,

and thus eliminating the reproduction distortion caused by the magneticnon-linearity of the magnetic circuit.

2 Claims, 6 Drawing Figures MAGNETICALLY ANISOTROPIC MATERIAL Pmmsum3,867. 587

FIG. la FIG. lb

PRIOR ART PRIOR ART I FIG. Ic PRIOR ART FIG. 2' PIC-5.3 M NETICMY 7 9 57 INSULATING IIIA TL IIZE I086 8 I0 I? s 579 '1 4\ W4 4 I 3 2 I 2 I FIG;4

MAGNETIC CIRCUIT FOR AN ELECTRO-ACOUSTIC CONVERTER CROSS-REFERENCE TORELATED APPLICATIONS This application is related in subject matter tocopending applications Ser. No. 315,962 and 315,963 filed Dec. 18, 1972,by Isao Yamamuro and assigned to a common assignee with the presentapplication. Application Ser. No. 315,963 has issued as U.S. Pat. No.3,830,986.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to a magnetic circuit for an electro-acoustic converter such asa speaker or a microphone.

2. Description of the Prior Art Normal magnetic material has a magnetichysteresis loop characteristic and the relation of magnetizing force andmagnetic flux density is non-linear. Therefore, when magnetic flux of avoice coil passes through a center pole and a plate near an air gap, themagnetic reaction in the voice coil results in a distorted output signalfrom the electro-acoustic converter such as a speaker or a microphone.

Accordingly, various methods eliminating the distortion have beenattempted, such as methods of covering a center pole a with a copper capb as shown in FIG. la, putting in a copper ring c as shown in FIG. lb,or making a depression on an upper surface of a center pole a andplating the depression with copper d as shown in FIG. 10. Theabove-mentioned methods shown in FIGS. la and lb, however, not onlyrequire a higher level of manufacturing technique than a conventionalnormal magnetic circuit for making the air gap a predetermined size, butalso results in decreasing the magnetic flux density within the air gapdue to the substantial increase of the air gap width. When using copperring c, the copper ring disturbs the magnetic flux distribution in theair gap thus disturbing the output signal. In the last case shown inFIG. lc, it is difficult to uniformly plate with sufficient thickness.

SUMMARY OF THE INVENTION The present invention has taken notice of theaforementioned problems and eliminates the distortion caused bythemagnetic circuit. The primary object of the present invention is toprovide a magnetic circuit for an electro-acoustic coverting devicecharacterized in that a portion or all of the magnetic circuit elementsdefining the air gap have magnetically anisotropic characteristics,thereby increasing the magnetic resistance against magnetic flux causedby a voice coil and eliminating the reproduction of distortion caused bythe magnetic non-linearity in the magnetic circuit.

Another object of the present invention is to provide a magnetic circuitfor an electro-acoustic converter which includes an air gap defined bymagnetic parts of magnetic material laminated with an insulating layerso as to have the magnetic anisotropy whereby the reproductiondistortion caused by the inductance of the coil is sharply reduced.

Another object of the present invention is to provide a magnetic circuitfor an electro-acoustic converter whereby an electro-acoustic converterhaving a high conversion efficiency may be manufactured withoutdifficulty because the eddy current power loss is decreased and most ofthe magnetic flux is used effeciently.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. la-lc are vertical sections ofconventional magnetic circuit portions for an electro-acousticconverter.

FIGS. 2 and3are vertical sections of embodiments of a magnetic circuitin accordance with the present invention.

FIG. 4 is an enlarged fragmentary sectional view schematicallyillustrating directions of magnetic flux of a voice coil.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Explaining one embodiment ofthe present invention in accordance with the drawings, 1 is a yoke, 2 isa center pole fixed in the center of the yoke l, 3 is a ringshapedmagnet, 4 is a plate fixed on the magnet 3, 5 is a voice coil cylinder,and 6 is a voice coil.

In the embodiments shown in FIGS. 2 and 3, the upper outside portion ofthe pole 2 and the inside portion of the plate 4 are respectivelyconstructed with magnetic parts 7 and 8, made of a stack of layers ofmagnetically anisotropic material such as two directional silicon steel9 interleaved with insulating layers 10 having comparatively highermagnetic resistance so as to have characteristics of magneticanisotropy. The easy magnetizing axis and hard magnetizing axis of theparts 7 and 8 are consequently directed at right angles and parallel tothe voice coil 6.

Therefore, the magnetic flux out of the magnet 3 returns to the samemagnet 3 by way of the plate 4, the easy magnetizing axis of themagnetically anisotropic part 8, the air gap, the easy magnetizing axisof the magnetically anisotropic part 7, the center pole 2 and the yokel. Namely, the magnetic flux for driving the speaker passes the air gapthrough the same path as in the conventional magnetic circuit. The totalmagnetic resistance of the magnetic circuit is slightly greater ascompared with that of the conventional magnetic circuit, because themagnetic flux for driving the voice coil passes through the easymagnetizing axis of the magnetic anisotropic parts 7 and 8.

The magnetic flux caused by the voice coil 6 follows the path shown bydotted lines in FIG. 4, that is, the magnetic flux completes closedloops which pass along both of the hard magnetizing axes of themagnetically anisotropic parts 7 and 8. However, the magnetic fluxpassing the magnetically anisotropic parts is limited because of thehigh magnetic resistance and the magnetic saturation brought about withthe small magnetic flux. Consequently, the reproduction distortion dueto the magnetic non-linearity of the magnetic circuit elements decreasesgreatly because of the weak mutual relation of the magnetic flux of thevoice coil 6 and the magnetic circuit parts. Further, since theresistance of the two directional silicon steel is higher than that ofnormal iron, silicon steel is useful for diminishing eddy current powerloss caused by the magnetic flux of the voice coil in the magneticcircuit elements. Further, the insulating layers 10 not only act asmagnetic resistance members, but are useful to decrease the eddy currentpower loss, and thus a speaker or a microphone having a high conversionefficiency may be produced without difficulty because the magnetic fluxacts to good advantage. Further, the sheets of magnetically anisotropicmaterial or normal magnetic material act as short rings, and thus thedistortion is reduced even more.

In addition, the functions and advantages such as mentioned above areachieved by laminating the sheets of the-magnetically anisotropicmaterial or the normal magnetic material using adhesive paste layers orinsulating paint layers.

As described above, the magnetic circuit of the present invention iscomposed of laminated magnetic parts which increase the magneticresistance against the magnetic flux caused by a voice coil. The sheetsof the magnetically anisotropic material are oriented such that thedirection of the easy magnetizing axis is the same as the magnetic fluxfor driving the speaker or the microphone within the air gap.Consequently, the reproduction distortion is drastically reduced. Sincethe insulating layers are between the sheets of the magnetic material,the eddy current power loss decreased, consequently a speaker or amicrophone having high conversion efficiency may be manufactured.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing'from the spirit and scope of theinvention.

What is claimed is:

1. An electro-acoustic converter comprising:

a. a center pole;

b. voice coil means surrounding said center pole;

c. magnetic means surrounding said center pole and said voice coilmeans; and

d. plate means fixed to said magnetic means; the improvement comprisinge. wherein at least a portion of said center pole is a stack of layersof a magnetically anisotropic material interleaved with insulatinglayers;

f. wherein at least a portion of said plate means is a stack of layersof a magnetically anisotropic material interleaved with insulatinglayers.

2. The apparatus of claim 1 wherein said at least a portion of saidplate means is all of said plate means. =l

1. An electro-acoustic converter comprising: a. a center pole; b. voicecoil means surrounding said center pole; c. magnetic means surroundingsaid center pole and said voice coil means; and d. plate means fixed tosaid magnetic means; the improvement comprising e. wherein at least aportion of said center pole is a stack of layers of a magneticallyanisotropic material interleaved with insulating layers; f. wherein atleast a portion of said plate means is a stack of layers of amagnetically anisotropic material interleaved with insulating layers. 2.The apparatus of claim 1 wherein said at least a portion of said platemeans is all of said plate means.